Plastic lenses for spectacles have various characteristics, which glass lenses do not have, in that they are lightweight, they are hardly broken and they are easily dyeable. Heretofore, acrylic resins, polycarbonate resins and ADC resins have been used to produce such plastic lenses. At present, urethane resins with high refractive indices are used to produce them, and plastic lenses with refractive indices of higher than 1.60 are sold in the market. Minus lenses with refractive indices of 1.60 or higher may be thinned even at their peripheries, and such thin lenses look nice. However, in order to reduce the weight of plastic lenses, it is still desired to further thin their center parts as much as possible, while removing the aberration to occur on their aspherical areas, thereby to further thin their peripheries.
As having good optical characteristics and other various excellent characteristics, plastic lenses are superior to glass lenses but the scratch resistance of the former is much lower than that of the latter. Therefore, for plastic lenses for use in spectacles, it is indispensable to coat their surfaces with a hard coat layer.
In order to improve their scratch resistance, plastic lenses may be coated with a hard coat layer comprising any of organic silicon compounds and epoxy resins. The hard coat layer of that type is very advantageous, when it is further coated with a thin metal film to form a non-glare layer thereon. However, the lens with the constitution of that type is seriously defective in that, if the hard coat layer and the non-glare layer are cracked on impact, the base lens will also be cracked often resulting in complete breakage of the lens. In general, if a plastic lens is directly coated with a hard coat layer and if the hard coat layer is over-coated with an inorganic layer of, for example, TiO.sub.2, ZrO.sub.2 or SiO.sub.2, through vacuum vapor deposition, the thus-coated lens shall have poor impact resistance and is easily cracked since the layers are very brittle though being hard. Therefore, it is said that, if such layers are simply formed on a plastic lens, the impact resistance of the coated lens will be lowered to 1/20 or less of the original impact resistance of the non-coated lens.
In order to ensure the reliable safety of spectacles for users, we, the present inventors have assiduously studied to develop plastic lenses for spectacles which have high impact resistance satisfying the requirements for commercial use and which are as safe as possible, and, as a result, have found that a plastic lens having a primer layer with a predetermined thickness or larger as formed between the base lens and the hard coat layer may have improved impact resistance.
In Japanese Kokai Patent Publication Nos. 63-87223 and 63-141001, disclosed are polyurethane primers to be obtained from a particular polyol or active hydrogen-having compound and a diisocyanate or polyisocyanate. These primers are applied to lenses of diethylene glycol bisallyl carbonate or its copolymer for the purpose of improving the adhesiveness between the lenses and the hard coat layers to be formed on the lenses. In the present invention, preferably used are primers of that type.
The thickness of the center parts of the lenses used in the above-mentioned prior art techniques is 1.6 mm or 1.2 mm. At present, however, recent lenses with high refractive indices may be further thinned with the increase in their refractive indices. For example, the thickness of the center parts of such lenses with high refractive indices may be from 0.7 mm to less than 1.2 mm. Accordingly, it has become possible to provide minus lenses having thin peripheries and plus lenses having thin center parts, which are fashionable. However, the reduction in the thickness of lenses inevitably results in significant reduction in the impact resistance thereof. Therefore, we, the present inventors have targeted the provision of lenses which may still have high impact strength even when their center parts are thinned in the manner as mentioned above.
The impact resistance of lenses is tested generally in accordance with the FDA Standard, for which a steel ball having a weight of about 16.4 g and a diameter of about 16 mm is dropped onto a lens from a height of 127 cm. In the American National Standard Institute (ANSI Z87.1 1989), stipulated is a method for testing safety spectacles for industrial use, in which a steel ball having a diameter of 25.4 mm is dropped onto a lens from a height of 127 cm. This is to ensure the reliable safety of spectacles for users.
To evaluate the impact resistance of lenses, employed is a test method of dropping a steel ball (16.4 g) as stipulated in the above-mentioned FDA Standard or a steel ball n-times heavier than the steel ball for the FDA Standard, onto a lens to be tested, and the lens thus tested is evaluated to have a degree of safety of "n-times of FDA". In this method, however, if the steel ball used is to have a large diameter, the contact area between the ball dropped and the lens being tested is to be large. Therefore, the data as obtained according to this method could not indicate the true impact resistance of lenses tested. We, the present inventors used, in place of this method, the BRUCETON method as approved by the U.S. OLA. In the BRUCETON method, we used the steel ball (16.4 g) as stipulated in the FDA Standard, while accelerating its dropping speed, and we obtained the striking energy needed for breaking the lens tested to evaluate the degree of safety of the lens. In the FDA Standard, the striking energy is defined to be 0.204 joules (J). Therefore, the impact resistance of the lens tested according to the BRUCETON method is represented by the multiple of said value, 0.204 J.
On the other hand, as has been so mentioned hereinabove, the formation of a hard coat layer is indispensable for plastic lenses. If, however, the refractive index of the base lens is larger than 1.60, the light reflected on the interface between the hard coat layer and the base lens interferes with the light reflected on the surface of the hard coat layer to often produce interference fringes. Theoretically, there will occur no problem if the refractive index of the base lens is the same as that of the hard coat layer. In fact, however, there is a difference of 0.1 or larger between them. It may be possible to prevent the formation of such interference fringes, if the hard coat layer is much thinned. However, the wavelength of visible rays varies within a range of from 400 nm to 700 nm. Therefore, even when the hard coat layer is thinned as much as possible, such is still problematic in that the thinned hard coat layer may be effective only in preventing the interference fringes from light having a certain wavelength but is ineffective in preventing the others from other light having different wavelengths, and is further problematic in that the thinned hard coat layer could not have satisfactory scratch resistance.
In Japanese Kokai Patent Publication No. 5-341239, we, the present inventors have disclosed the advantages of the plastic lenses for spectacles with high refractive indices which we have commercialized. After this disclosure, we have further studied to improve and enhance the quality and the producibility of various plastic lenses for spectacles, and have now completed the present invention. Specifically, the present invention is to provide lenses for spectacles, which have high impact resistance and which produce few interference fringes. In addition, according to the present invention, the latitude in the acceptable modification of conditions for surface treatment of lenses is enlarged, whereby the producibility of lenses is, after all, much enhanced.